Telephone system



Feb. 27, 1945. G. HEcHT TELEPHONE SYSTEM Filed Oct. l5,

ATTORNEY atented Feb. 27, 1945 UNITED s'rATEsy-PATENT OFFICE TELEPHONE VSYSTEM George Hecht, AstoriaN."Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 15, 19.41, Serial No. 414,997

4 Claims. l (,lCl. 178-44) This invention relates totelephone systems and particularly to the receiving equipment of a transmission line over which alternating currents, preferably within the Voice frequencyrange, are transmitted for effecting the operation rof sai-d equipment to control, in turn, the selective positioning of switching apparatus. More particularly, the invention has to do with means for controlling the degree of sensitivity-response of the detector channels of the receiving equipl ment of the telephone system shown and described in Patent 2,237,742, issued to A. A. Lundstrom on April 8, 1941, and utilized as part of the telephone system disclosed in my Patent 2,260,318, issued on October 28, 1941. In said last-mentioned patent the receiving channels of the telephone system are each adapted to respond `toga particular frequency in an incoming signal composed of several frequencies, and an' enabler circuit is provided which is adapted to activate" the detector elements of said channels for response to the signal `some time after its arrival in order to avoid possible false operation of the channels from the transient disturbance set up by the initial impact of the signal energy upon the ltering devices immediately preceding the detector elements of the channel. Since theduration of the disturbance depends `upon the intensity of the signal which, in turn, depends upon the length of the transmission loop, the operation of the ,enabler circuit disclosed and claimed in said Patent 2,260,318 depends upon the applicaoperatelsensitivity of the `several detector ele-- yments may be adjusted for the maximum loop for which the negative bias to the grid electrodes of said elements kwill be a minimum when the direct current in .the -loop is avminimum. For

the minimum loop when the direct vcurrent is a maximum, the just-operate sensitivity of the-detectors-may then be maintainedto approximately the same value as before by applying to the grids :thereof Athe proportionatelygreater negative bias derived from ther loop direct current. This and other .features of the invention will ,be more readily ascertained from .the following .detailed description of said invention, appended claims and lattached drawing in which is shown schematically certain elements of 'the telephone system completely disclosed .in said above-mentioned Patent 2,260,318 and certain other elements to which my invention more directly relates. Y

Referring now .to the drawing, X represents a subscribers ystation in an automatic telephone system, said station being equipped with adevice I suitably arranged to produce .one or more currents of different `frequencies in the voice frequency range in response to the .depression of keys 0r the like that represent digits or other `characters making up the Wanted designation, .arid2V .represents a line .extending from the station instrument to .the central oiice and estab- 4lished therebetween by any suitable means such as, for instance, by line extending switches and tion to the grid of a vacuum tube electronic de- 4vice of a marking potentialwhich is determined by a direct current flowing in the loop, the curafter the arrival of the signal, but the amount of sensitivity of the several elements is controlled by the application to the grid elements thereofl of a negative potential derived from the quantityy of direct current flowing over the loop. By

means of this control, the 'length of the Vloop over which the signal current can be transmitted may be considerably increased inasmuch as the justthe like. .The conductors of said line extend to battery and ground serially through the upper and lower windings of relays 40| and 403, respectively, and ,resistances 38 `and 31, respectively, with rectiiier 442 bridged across the upper wind- -ing'or relay 403 and rectier 443 bridged across the lower winding of said. relay. The line, for the purpose of switching control and further eX- tension, is connected to a free sender through one of a plurality of selectable sender connector relays i3, said sender comprising coupling trans- "formers 3, 4, i5 and 6, amplifiers .8 and 9, and the enabler circuit I0, the latter functioning to sensitize detector tubes a, b, c, d and e of the channels fl, f2, f3, -fd and f5 which are individually so designated to agree with the frequency notation of thefrequency to which each of the ,channels is respectively assumed adapted to respond, which channels include, also, .individually A responding channel relays A-E,`and the settable register relays (not shown), the latter following the operation `of the channel relays in order to permanently register the operation of the same when said channel relays operate to signalize an incoming impulse. The amplifiers 8 and 9 are commercially available circuit units while the channels fI-f are completely disclosed in the above-mentioned Patent 2,237,742 to A. A. Lundstrom to which reference is made for a more complete understanding thereof, while the register relay circuit may be of the type disclosed in said patent or of any other suitable design.

VIt is to be understood, however, that the invention is not limited to channels of the type described in the last-mentioned patent or to other apparatus herein used for purposes of illustrating the 'operation of the invention.

The enabler III, the operation of which is com pletely described in Patent 2,277,228, issued on March 24, 1942, comprises the varistor bridge I6 and the polarized relays II, I8, I9 and 20 with their intermediate capacitor-resistance networks controlling the operating times of Said relays in the manner set forth in said last-mentioned patent.

In the operation of the invention, a call is initiated at station X by the removal of the station instrument from the switchhook, whereupon the line is then extended by any suitable means to the central office at which an available sender is then connected to said line through the medium of a sender connector I3 after which key tone is transmitted by a suitable circuit (not shown) back to the calling station X as `a signal that the sending device I may be operated to transmit the number of the wanted station, all in the manner described in the above-mentioned Patent 2,237,742 to A. A. Lundstrom.

When any one of the ten keys of the sending device I is operated, two out of the ve dened frequencies ,I-f5 are generated in the combination called for by a suitable code to designate the digit represented by the key depressed, and the resulting current produced thereby is transmitted over a circuit which may be traced from one side of the sending device I, over the lower conductor of the looped pair 2, the lower back contacts of relay I2, the next-to-the-inner contacts of sender connector cut-in relay I3, the primary winding of transformer 3, the inner contacts of relay I3, the top back contacts of relay I2 and the upper conductor of looped pair 2 to the other side of the station sending device I. The current composed of the two frequencies is induced in the secondary winding of transformer 3 and fed into the input side of amplifier 8 whence, after suitable amplification, it is applied, through transformer 4, to the input pad I4 from which, after passing through suitable band-pass filters BPF- it is made available for operating the channel vacuum tube detectors w-e when the cathode elements of the same are activated by the enabler I0 as explained hereinafter.

Considering, now, that portion of the signal energy which is transmitted through transformer 6, it will be observed that the enabler I0 comprises the varistor bridge I6 and the four polarized relays I'I, I8, I9 and 20, each of said relays having a permanently closed polarizing circuit through their respective lower windings that causes their respective armatures to assume the positions indicated in the drawing. Between each two relays, beginning with relay I1 and ending with relay 20, there is an intermediate condenser-resistance network which joins a contact of the relay on the left with the upper winding of the relay'on the right. Considering relays I 'I and I8, for example, the network made up of resistances 22 and 23 and condenser 2I joins the right contact of relay I1 with the upper winding of relay I8. Since the armature of relay II is connected to the positive battery 30, condenser 2I is normally charged to the potential of this battery. Condenser 24 of the network between the left contact of relay I8 and the upper winding of relay I9, on the other hand, is normally at ground potential since one side of it is permanently connected to ground while its other side has ground applied to it through the armature and left contact of relay I8. Also, condenser 29 of the network between the contacts of relay I9 and the upper winding of relay 20 is charged to the potential of the positive battery 30 through resistance 28 and the upper winding of said relay 2U since the short-circuiting ground path for condenser 29 is normally open at the right contact of relay I8, which fact leaves condenser 29 exposed to the charging circuit that traces from the positive pole of battery 30 through the upper winding of relay 20, resistance 28 and condenser 29 to ground.

Assuming, therefore, that relays II, I8, I9 and 2U have their respective armatures in the positions shown and that condensers 2|, 24 and 29 are charged, discharged and charged,- respectively as indicated above, ground is normally disconnected from conductor I5 at the right contact of relay 20, which conductor is, in turn, connected to the cathodes of the channel detector tubes a/-e of the several channel detectors fI-f5 and causes them to be normally unresponsive to the signal energy available at resistance pad I4.

The enabler I0, the elements of which have been described above, is identical with the enabler shown and described in Patent 2,277,228 to which reference has already been made, and the elements of the present enabler have been given designations identical with those of the enabler in said patent in order that, in the present specification, the operation of the enabler set forth in said patent may be followed without any additional description herein. Briefly recapitulated, however, it is the function of the enabler I0 to respond to the signal energy incoming over transformer 6 and to operate in consequence thereof relays I1, I8, I9 and 20 sequentially in a certain timed relation whereby, after a predetermined interval subsequent to the time when the signal energy is first applied to transformer 6, relay 20 is operated to apply ground to conductor I5 and continues to apply ground to said conductor for a definite interval thereafter subsequent to which ground is removed from said conductor (by the release of relay 20) even though signal energy is still available at transformer 6. The manner in which this is accomplished is immaterial to my present invention, and is for the enabler illustrated, fully described, as said before, in Patent 2,277,228. Suffice it to conclude that when relay 2U operates,

ground is applied to conductor I5 which, being.

connected to the cathode of each of the several detectors a-e, causes said detectors to become activated for response to signal energy available at resistance pad I4.

I'he extent, or the degree, of sensitivity with which the several detectors will respond to the incoming signal will depend, however, upon the value of the biasing potential which is applied to their respective grids. That is, the quantlty .of -current that will flow kin .the cathode-anode circuit-of each of thedetectortubes `for'operatling their respective channel relays A-l-E willdevpend, as is well-known, upon the value of a negative potential bias applied to their respectivegrid electrodes. The positive components of the sigynal energy applied to these grids will, of course, iincrease the cathode-anode current and, for the longest line loop, `will be sufficient to increase this :current to a value that will cause the correspond- .ingchannel relay to operate.l It is evident,how- -ever, ythat if the cathode-anode current is just '-sulicient, on the longest loop, to cause the operation of the channel relays then, on the shortest `or zero loop,'this current willbe considerably increased inasmuch as the positive bias thenap- .plied to the gridswill be a maximum. vSince the wave front of an incoming alternating current .signal will, when striking `the input side of irn- .IEPF/ produce a transient .voltage on the output rside thereof that will be applied to the gridof .each of `the detector tubes, thenthose tubes which lie on either side of the tubes in the channels that pass the frequencies of the signal may falsely if respond and falsely operate their respective channel relays. In order to eliminate the possibility of this false operation it is necessary, therefore, to neutralizethe positive grid voltage applied to all tubes by the transient voltage set up by the signal, by a negative potential that varies with the length of the loop so that the resultant bias will always be no more than suii'icient to cause the operation of only those tubes in the channels that pass the frequencies of the signal. This potential is applied over conductor l l which, on the one side is connected, through the middle set of contacts of relay I3, to one side of resistance 31 and, on the other side, is connected to the impedance network in each channel that extends to the grid of the detector tube thereof over the secondary winding of the transformer intermediate the band-pass filter (BPFfI of channel l, for instance) and the grid.

Now when the line loop is established, a direct current circuit is completed which may be traced from ground, through resistance 31, the lower windings of relays 403 and 411i, respectively, the lower back Contact of relay I2, over the loop, the top back contact of relay l2, the upper windings of relays im and Q03, respectively, and through resistance 38 to' negative battery. The current flowing through this loop is, of course, determined by its ohmic resistance the only variable element of which is the resistance of the loop itself. Since conductor Il is connected to the line terminal side of resistance 31, a negative potential is available thereat which is applied to the grid of each of the detector tubes. The quantity of current which ilows through the cathodeanode circuit of leach tube is then effected by the value of this negative potential which, in turn, has a value determined by the length of the loop. The value of this potential, prior to the transmission of any of the signal impulses, is practically unaffected by the alternating current signal potential superimposed thereon when the signal impulses are transmitted inasmuch as the former is preponderately large in comparisonto the unamplied signal voltage which is similarly superimposed thereon. Hence the value of the biasing potential derived from the direct cur-rent flow is substantially the only potential applied to the grid of each tube, that will determine the sensitivity yample to cause the operation of the relay of "a .channel which passes a frequency .of the :signal energy, yet because conductor H extendslto .the vgrids of all the tubes, .the `heavy negative bias applied to the tubes of those channels "which do :not pass the signalfrequency, protects theseftubes 'from falsely voperating on the transient disturbancesetup in adjacent channels bythe impact ofthe signal current upon all the channels. A0n va maximum loop, on the other hanchthe loop currentis a minimum, the negativepotential de rived from the loop and applied to the` grids of all the tubes is a minimum and the tubes will .be at theirmaximum sensitivitylin orderito per- .mitfull response onrthe weak signal current available for operation. Although, in this case, the transient disturbance is also produced, it is of considerably diminished intensity and the small negative bias applied to the grids of all the loops will be sui/iicient to protect-the tubes which `are adjacent to those responding to the signal current. Thus, the sensitivity of the detectors is varied in accordance with the length of theloop to provide for appropriate response of the particular channels involved to the frequencies of the incoming signal current, and to protect adjacent channels against false operation in consequence of the transient disturbance produced by the signal.

When the signal impulse is over (assuming the correct depression of a key at device I), relay Il will release and, according to operations described in my above-mentioned Patent 2,277,228, initiates a series of operations by which the relays of the enabler l0 are restored to their original positions. Since, by this time, relay 20 has already released to remove ground from conductor l5, the circuit is normally in readiness to receive the next signal when the operations described above are repeated.

While I have described my invention and the means for utilizing the same in connection with a specic embodiment thereof, it is to be understood that various other embodiments thereof may be made by those skilled in the art without departing from the spirit of the invention as defined within the scope of the appended claims.

What is claimed is:

1. In a telephone system in combination, a transmission line provided with a plurality of channel detectors adapted for response to alterhating current signals, a source of direct current connected to said line, and means connected to of the tube response to the signal current. On

said line and to said detectors for controlling the sensitivity-response of said detectors to incoming alternating current signal energy in accordance with the intensity of the direct current owing through said line and through said means.

2. In a telephone system in combination, a transmission line provided with a plurality of channel detectors each including an electrode adapted for response to alternating current signals transmitted over said line, an enabler for said channel detectors adapted to activate the same for response to said alternating current signals, a source of direct current potential for connection to said line, and means connected between said line and said electrodes4 for applying a negative potential to said electrodes which is derived from the direct current ilowing over said line whereby the sensitivity of detector response to said alternating current signals is controlled by the value of said negative potential.

3. In a telephone system in combination, a transmission line provided with a plurality of channel detectors adapted for response to alternating current signals, a source of direct current potential connected to said line, an enabler for activating said channel detectors for response to said alternating current signals at a time subsequent to the arrival of said signals, and means effective in response to the direct current owing over said line from said source of direct current potential for applying a control potential to each of said detectors in advance of said alternating `current signals whereby the sensitivity response channel detectors each including an electronic device provided with a grid electrode adapted to have applied thereto a positive potential derived from alternating current signals transmitted over said line, and a cathode-anode circuit the current ow through which is dependent upon the voltage applied to the grid, a source of direct current potential connected to said line, an enabler for activating all of said electronic devices for Vresponse to said alternating current signals in consequence of the voltage derived therefrom for application to the grid electrodes of said electronic devices, a conductor extending between 

